1. Field of the Invention
The present invention relates to the field of audio electronics, and in particular to electrical interconnect cables.
2. Description of the Related Art
High-bandwidth, low-loss analog pairs, quads and twin-axial type interconnects have been in use since the advent of television. To achieve high-bandwidth and low-loss, conductors were initially spaced apart to minimize capacitance and suspended in an air dielectric. Disc-shaped spacers with holes in them were typically oriented radially in an insulating tube and the conductors passed through them forming a twisted pair or twisted quad construction. To minimize dispersion effects and losses, bare conductors were often used, particularly for video and RF transmission. The bare conductors were suspended within semi-rigid tubing. Flexible tubing could not be used to house the conductors because when the interconnect was sharply flexed, the bare conductors would short to each other. The semi-rigid jackets that were required made these interconnects difficult to handle and failure-prone. Constructions of these types are described in Hultman, U.S. Pat. No. 680,150 (1901), Markuson, U.S. Pat. No. 2,188,755 (1940), Curtis, U.S. Pat. No. 2,119,853 (1935) and U.S. Pat. No. 2,034,026 (1936), Green, U.S. Pat. No. 2,034,033, and Cogan, U.S. Pat. No. 4,954,095 (1990).
An alternate construction is taught by West in U.S. Pat. No. 716,155, which also suspends bare conductors in a semi-rigid tube. West teaches that an accordion-folded insulating strip with holes or slots in it can form a supporting structure for bare conductors. The disadvantage of this folding structure is that it only flexes in one dimension. Lead-coverings are described as jacketing such a cable to shield it and to prevent it from flexing.
With the advent of low-dielectric constant materials such as Teflon and foamed polymers, bare conductor designs were eventually replaced with conformally insulated wires in both coaxial and twin-axial constructions. The conformally insulated wires were more durable, allowed flexibility and eliminated the shorting hazard. Unfortunately, these new materials did not perform quite as well as the bare conductors in air dielectric because their loss and dielectric absorption characteristics were inferior. These characteristics cause the transfer function of the conductors to vary depending upon amplitude and frequency, thereby degrading the signal quality. Dielectric absorption can cause smearing of signals at all audio frequencies due to latent storage of charge in the dielectric. For most video, RF transmission and some digital transmission applications however, the performance of these insulators was sufficient and interconnects could be manufactured at much lower cost. For these reasons, insulated conductors find wide use in electronic signal transmission.
With improvements in audio media, amplifying equipment and loudspeakers for music and theater sound reproduction, the need has increased for high-performance interconnects that can resolve these more accurate signal sources. High-performance interconnects must have low dielectric absorption, low capacitance and low dielectric losses. These goals are all achieved with constructions that comprise bare conductors separated by air dielectric. Such constructions have the effect of substantially improving multi-channel image focus and dynamics at all audio frequencies, creating a more live audio reproduction when compared to insulated conductor designs.
The need for higher performance interconnects has been addressed by several audio interconnect designs that have attempted to approach the optimum configuration of bare conductors suspended in air dielectric. One of these designs utilizes a flexible insulating tube. Two bare or insulated conductors are wrapped around the tube in a xe2x80x9cbarber-polexe2x80x9d fashion, with interstitial small tubing or fillers wrapped between the two conductors to keep them spaced apart. This construction further requires insulating materials surrounding the conductors to hold them tightly against the outside of the tube and spaced from each other so they do not move when the tube is flexed. These materials for spacing and holding negate the positive effects of having bare conductors. One such construction is described in Low, U.S. Pat. No. 4,997,992.
A second construction involves an extruded insulating tubing, which has one or more smaller tubes, which are integral to the extrusion and inside the larger tubing. The smaller tubes house bare conductors. This construction has the disadvantage that the conductor or conductors must be xe2x80x9cfishedxe2x80x9d through the smaller tubes and these can be quite long. Also, the close proximity of the conductors to the small surrounding tubes will increase dielectric absorption and loss.
A third construction involves disc-shaped spacers, which suspend the conductors within an insulating tube as described in Nugent, U.S. Pat. No. 5,880,402. One conductor between each adjacent pair of spacers is insulated, achieving a half-insulated interconnect. Since one of the two conductors is insulated between each adjacent pair of spacers, the two conductors cannot short together. This construction has the disadvantage of having insulation on half of the conductors, which will cause higher dielectric absorption and loss when compared to bare conductor constructions.
Some of the described twin-axial and suspended-pair constructions are definitely an improvement over fully insulated conductors, but they still suffer from audible dielectric absorption effects.
The present invention finds application in the field of high-fidelity audio, and particularly to digital and analog audio interconnect cables. The present invention is an interconnection cable that can be used for balanced or single-ended analog signal transmission or digital single-ended or differential signal transmission. The invention includes several flexible constructions that suspend bare conductors in a primarily air dielectric while eliminating the shorting hazard between the conductors.
A first single-ended, twisted-pair interconnect and the preferred embodiment, according to the present invention, begins with a first conductor which is bare or uninsulated and a second conductor which is also bare or uninsulated. Alternatively, the first and/or second conductor can include a conformal or other insulation along all or part of their length. An insulating tube which has uniform perforations along its length forms the supporting structure for the interconnect. The first bare conductor is woven through the perforations in the tube forming a xe2x80x9csquare-wavexe2x80x9d pattern, which is aligned along a first radial line intersecting the radial center of the tube. The second bare conductor is woven through the perforations in the tube forming a xe2x80x9csquare-wavexe2x80x9d pattern along a second radial line, which is 90 degrees rotated from the first radial line and shifted along the longitudinal axis of the tube. This construction creates a twisted-pair geometry that locates a minimum of insulating material in contact with and between the two conductors. There is no shorting hazard between the two bare conductors because of the spacing created by the woven pattern. The spacing of the two bare conductors when woven through the insulating tube minimizes the interconnect capacitance.
A balanced or differential twisted-pair interconnect according to the present invention is constructed identically to the first single-ended, twisted-pair cable, but with the addition of a third insulated conductor located inside the tube and extending the length of the tube. This provides the xe2x80x9cgroundxe2x80x9d conductor that is necessary in a balanced or differential connection. Because this conductor is contained inside the tube, it becomes centered within the tube due to contact with the other conductors. The ground conductor is orthogonal to the other conductors, minimizing its effect on the performance of the interconnect, by reducing coupling with the bare conductors.
A second single-ended, twisted-pair interconnect according to a preferred embodiment begins with a first conductor which is bare or uninsulated and a second conductor which is also bare or uninsulated. Alternatively, the first and/or second conductor can include a conformal or other insulation along all or part of their length. An insulating tube which has perforations along its length forms the supporting structure for the interconnect. The first bare conductor is woven through the perforations in the tube forming a xe2x80x9ctrapezoidal-wavexe2x80x9d pattern, which is aligned along a first radial line intersecting the radial center of the tube. The second bare conductor is woven through the perforations in the tube forming a xe2x80x9ctrapezoidal-wavexe2x80x9d pattern along a second radial line which is 90 degrees rotated from the first radial line. This construction""s bare conductors and twisted-pair geometry locates a minimum of insulating material in contact with and between the two conductors. There is no shorting hazard between the two bare conductors because of the spacing created by the woven pattern. The trapezoidal pattern has the additional advantage of using less conductor length to span the length of the insulating tubing.
A balanced or differential twisted-pair interconnect according to the present invention is constructed identically to the second single-ended, twisted-pair cable, but with the addition of a third insulated conductor located inside the tube and extending the length of the tube. This provides the xe2x80x9cgroundxe2x80x9d conductor that is necessary in a balanced or differential connection. Because this conductor is contained inside the tube, it becomes centered within the tube due to contact with the other conductors.
The twisted-pair cable assemblies can be surrounded by an insulating jacket to prevent shorting to the bare conductors, which is corrugated to minimize contact of the jacket with the bare conductors. Alternatively, an insulating jacket can surround the woven cable assembly, which has internal ribs to minimize contact of the jacket with the bare conductors.
If a metallic shield is applied to surround the corrugated jacket or the jacket with internal ribs, the jackets will create a space between the bare conductors and the metallic shield, reducing interconnect capacitance over conventional constructions. The surrounding metallic shield may consist of woven wires or metallic foil or both.
Multiple pairs of bare conductors, each woven through an insulating tube can also be combined in parallel to form a digital or analog interconnect cable. Component electrical characteristics and length of the interconnect along with the frequency range of interest affect the choice of the number of conductors and the conductor wire gauge required to optimize the quality of signal transmission. To optimize the high-frequency response of an analog interconnect, the wire gauge is limited to about 20 AWG due to skin-effect. Improved high-frequency response will generally be achieved with smaller diameter gauges (22-26 AWG). To optimize the bass response and dynamics of the interconnect, a sufficient number of pairs must be connected in parallel to achieve a low inductance. A two-pair construction can include four bare conductors that are woven through an insulating tube, each having a square-wave pattern.
The constructions of the present invention will become more apparent from the following detailed description in conjunction with the accompanying drawings.